WASHINGTON (AP) — Defense Secretary Chuck Hagel said Friday he sees no simple answer to why the U.S. military is suffering so many ethical lapses, but he vowed to get to the bottom of a trend of embarrassing disclosures.

He told a Pentagon news conference that even with an accumulating tide of reports of generals and admirals abusing their positions and facing an array of accusations of unethical behavior, the moral fiber of the military remains strong.

"But some of our people are falling short of these high standards and expectations," Hagel said.

Just in the last few weeks the Air Force and the Navy have disclosed investigations of cheating on tests by officers and sailors involved in nuclear operations. The Air Force also is pursuing a drug use investigation, and sexual assault and harassment cases have surfaced across the services.

"Competence and character are not mutually exclusive," Hagel said. "They are woven together. They must be. And an uncompromising culture of accountability must exist at every level of command."

Hagel said he doesn't know the full dimensions of the problem, which has been growing in visibility for more than a year and was a point of emphasis by his predecessor, Leon Panetta.

"I think we need to find out, Is there a deep, wide problem? If there is, then what's the scope of that problem? How did this occur? Was it a constant focus of 12 years on two long land wars, taking our emphasis off some of these other areas? I don't know. We intend to find out," Hagel said.

Hagel said he would assign a senior general to his staff soon to coordinate with the military services their plans for attacking the problem of ethical lapses.

Separately, Hagel announced that President Barack Obama has nominated Robert Work to be deputy secretary of defense, replacing Ashton Carter. Carter resigned in December. Work is a retired Marine officer and currently the chief executive officer of the Center for a New American Security, a Washington think tank.